Forming thermoplastic articles



Dec, 8, 1970 5. cHNsb V 3,546,221

magma THERMOPLASTIC ARTICLES Filed Juna23, 1969 2 Sheets-Sheet 1INVENTOR:

HERBERT s. JOHNSON BAY ms ATTORNEY DecfiS, 1920 Filed June 23, 1969 H.G. JOHNSON FORMING THERMOPLASTIC ARTICLES 2 Sheets-Sheet 2 FIG. 4

FIG. 6

F l G. 8

INVENTOR:

HERBERT G. JOHNSON HIS ATTORNEY United States Patent 3,546,221 FORMINGTHERMOPLASTIC ARTICLES Herbert G. Johnson, Havertown, Pa., assignor toShell Oil Company, New York, N.Y., a corporation of DelawareContinuation-impart of application Ser. No. 561,871, June 30, 1966. Thisapplication June 23, 1969, Ser. No. 835,607 The portion of the term ofthe patent subsequent to Sept. 30, 1986, has been disclaimed Int. Cl.B29d 7/14; B32b 31/20 US. Cl. 264249 18 Claims ABSTRACT OF THEDISCLOSURE A method is disclosed for the shaping of plastic sheet intoshaped articles of moderate draw depth. The method is suitable for largescale production of thick-walled thermoplastic items. It consists offorming a thermoplastic sheet at a temperature substantially below thatat which it can be thermoformed, by forcing a thick body of a yieldablemass, such as a body of elastomer, into contact with one surface of thesheet, opposite a non-yielding die. The sheet is deformed under tensilestress as the mass tends to move with the sheet. The sheet is held incontact with the die until stress is substantially relieved. The formedarticles conform substantially to the shape of the die, exhibitinglittle or no shrinkage or springback.

This application is a continuation-in-part of US. Ser. No. 561,871,filed June 30, 1966 now Pat. No. 3,470,291 granted Sept. 30, 1969.

This invention relates to a method of forming thermoplastic articles.More particularly, it relates to a method for shaping sheets or platesof thermoplastic polymers into shaped articles by a simple formingoperation at temperatures below those at which such sheets or plates canbe thermoformed.

The term sheet generally implies a maximum thickness of about inch. Tosimplify the disclosure and claims, the term sheet is defined herein asa flat article having a substantially uniform thickness of at least 0.01inch and extending substantially in the two other dimensions, e.g.,having its length and breadth each at least five times the thickness.The thickness may be as much as two or three inches. Sheet thuscomprises some articles conventionally designated film as well as thoseconventionally designated plate.

In essence, the method of this invention comprises forming an articlefrom sheet of thermoplastic polymer by shaping such sheet between asubstantially rigid die and a confined body of yieldably deformablematerial, such as rubber, while said sheet is at a working temperatureabove its glass transition temperature and below the thermoformingtemperature as hereinafter defined.

One mode of practicing this invention comprises placing a sheet ofthermoplastic material which is at a working temperature in theabove-defined range into contact with a non-yielding die and forcing aconfined body of yieldably deformable material, such as a rubber pad,against substantially the entire area of the surface of the sheetopposite the die with suflicient force to cause the sheet to conform tothe shape of the die, retaining the sheet and die in forced engagementfor a sufiicient period to permit the stress in the formed article to berelieved, typically a period in the range from 1 second to about 1minute, and releasing the formed article from the die. Articles formedin this manner have dimensions which are substantially unchanged fromtheir dimensions while in forced engagement with the die.

The method of this invention is particularly advantageous in forming ofarticles from thick sheets, e.g., in thicknesses of /2 inch and up,since it does not involve a prolonged cooling period as part of theshaping step, unlike other methods which are in commercial use today.

Another particular advantage of the method of this invention is that itpermits forming of articles having multi-layer walls of thermoplasticpolymer, and especially those composed of layers of different polymers,which cannot be produced by thermoforming methods in the usual casewhere the softening or melting points of the polymers differsubstantially.

The method of this invention is adapted to large scale production ofthick-Walled thermoplastic items, such as appliance lids and housings,corrugated panels, luggage shells, and so forth.

A great variety of methods for forming articles from thermoplasticsheets have been developed and are in use today. These are essentiallyall modifications of the socalled thermoforming method in which plasticsheet is heated to a thermoforming temperature at :which it can bereadily deformed under atmospheric or similarly low mechanical orpneumatic pressure, and is then forced into or over a mold or die byvacuum or slight pneumatic pressure, often with a mechanical plugassist. The formation of thermoplastic articles from sheet bythermoforming methodsincluding vacuum formingis characterized by thefact that sheet is shaped at a thermoforming temperature at which thepolymer acts essentially like a liquid and provides no substantialresistance to flow even at very moderate pressures such as atmosphericpressure, except where frictional resistance must be overcome. At suchtemperatures, thermoplastic sheet drapes or sags under its own weight.When an article is formed at this temperature, it must be substantiallycooled before it can be removed from the mold. Cooling must becontrolled to avoid warping or internal defects. Accordingly, suchmethods tie up expensive equipment for the period of time required tocool the polymer carefully to a temperature at which it is suificientlyrigid to retain its shape on removal from the mold. Time requirementsincrease rapidly with increasing thickness of the molded article. Atypical time cycle in vacuum forming of polypropylene sheet of milthickness is 1% minutes.

A complicating factor in the shaping of thermoplastic materials at hightemperatures is the fact that there is a substantial contraction of thematerial between its melt temperature and room temperature. As a result,the finished shaped articles do not usually conform to the precise shapeof the mold, and a change in dimension must be taken into account whenproducing the mold.

Other widely used methods of forming. thermoplastic articles, e.g.,injection molding, blow molding, and extrusion, also involve conversionof the polymer to a melt and resultant cooling requirements which add tothe complexity and expense of the equipment or result in excessivelylong time cycles for making thicker articles.

In the method of this invention, articles are rapidly formed fromthermoplastic sheet at temperatures below the above-definedthermoforming temperature of the polymer. For the sake of convenience,forming at these lower temperatures will be referred to herein ascoldforming, even though the polymer being formed may be at an elevatedtemperature close to, but below, its thermoforming temperature.

In developing the method of this invention it was found thatcold-forming of thermoplastic sheet, and particularly of polypropylenesheet, is attended by unexpected difficulties. In attempts to performsuch cold-forming on sheets of polypropylene having thicknesses of inchto A; inch or more between matched rigid dies, it proved impossible toform articles conforming substantially to the shape of the dies due to alarge degree of springback of the plastic sheet, e.g., exceedingpercent. springback is the difference between a given dimension of theshaped article and of the die in which it is shaped, after taking intoaccount thermal shrinkage.

Another problem in cold-forming of plastic sheet is that the plastictends to wedge between two closely spaced rigid surfaces such as thepunch and die, resulting in tearing or scuffing of the resultingsurfaces. The closer the clearance, the greater the difiiculty inmaintaining material flowespecially where such clearance progressivelydecreases in the direction of flow. Due to the slick nature of theplastic surface, any lubrication originally applied to the sheet or tothe tools may break down under pressure, causing the plastic to tend tostick to the tools and to tear or scufr'. This in turn builds upincreased resistance to flow.

A characteristic of polypropylene and other plastics which contributesgreatly to the difficulty of cold-forming is high compressibility. In anattempt to shape polypropylene between relatively moving metal surfacessuch as a punch and die, any resistance to flow such as friction resultsin a corresponding buildup of back pressure, which in turn appreciablycompresses the plastic. This causes a further increase in wedging actionand resistance to fiowso that when the pressure is released the plastictends to spring back violently or even to blow apart.

The method of this invention overcomes the abovementioned disadvantagesof cold-forming between matched non-yielding dies. It was surprisinglyfound that 0.01 to 1 inch thick sheets of thermoplastic material such aspolypropylene, polystyrene or ABS, cold-formed according to thisinvention between a metal die and a pad of yieldable elastomericmaterial, can be readily shaped to conform precisely to the die withoutspringback or with insignificant springback in very short formingcycles, typically in the range from 1 second to '1 minute, depending onthe thickness and temperature of the sheet, and with little or nowhitening of thinned parts.

Unlike metal dies in cold-forming of plastic sheets, the yieldable diemass tends to move with the plastic surface with which it is in contact;consequently there is relatively little frictional stress developed.Because only one inflexible metal surface is engaged by the plasticsheet, there is no chance of wedging, jamming or gallingwith resultanttearing and scufiing. Because there is little resistance to flow due tofriction, wedging or galling, there is very little buildup ofcompression stress. It is believed that this is the major factor towhich reduced springback in the formed article must be attributed.

A major difierence between the process of this invention and either thethermal sheet-forming methods of the art or the method of forming sheetbetween matched metal dies is that the use of a die of yieldablematerial in accordance with this invention results in shaping of theplastic sheet under tensile stress. This results in a unique advantageof this invention, but any substantially thinned sections show astrengthening effect of orientation. Since the thinned portions areusually the outer edges and corners of the article, which are exposed tothe most substantial stresses during use, it is desirable that they bestronger than the rest of the article, rather than weaker, as they arein thermoformed articles.

The use, as a die, of a yieldable material such as rubber also resultsin certain known advantages not attainable in forming between metaldies. Thus, reverse curved shapes and re-entry dimensions can be createdwhich would be impossible with any other than a fluid forming method. Ingeneral, most relatively thin plastic shapes can be snapped off or outof a metal form having such dimensions without necessity of providingcollapsing or split tools, because of the elasticity of such plasticarticles.

Broadly stated, this invention permits shaping sheetstock ofthermoplastic polymer in any thickness into shapes comparable to thosemade by thermoforming. Compared to thermoforming it has a much fasterforming cycle, especially for thick sections; reduces the amount wastedas trim; results in improved impact strength of thinned out sectionswhich, in thermoformed articles, are weaker than the body of the articlebut in the method of this invention are stornger; and permits formingcomplex shallow draws without wrinkling due to sheet sag.

Accordingly, it is the primary object of this invention to provide asimple, rapid, economical method for forming commercially usefularticles from sheets of thermoplastic polymers.

A more specific object is to provide a method for converting sheets ofthermoplastic polymers which result in excess spring-back when shapedbetween rigid dies at temperatures below their thermoforming temperatureinto articles which conform in dimensions substantially to the formingdie.

A further object is to provide methods for forming useful articles fromrelatively thick sheets of thermoplastic polymers in cycle times rangingfrom one second to one minute.

Further objects of this invention will appear from the followingdescription thereof, which is made in part by reference to the drawingwherein:

FIG. 1 is a simplified view, in vertical section, of apparatus employedin carrying out one mode of the process of this invention, utilizing arigid male die member and yieldable female die member, showing plasticsheet in position for molding prior to engagement of the die members;

FIG. 2 is a view of the apparatus of FIG. 1 while the die members are infull engagement;

FIG. 3 is a View of the apparatus of FIG. 1 after the die members havebeen disengaged and the molded article released from the die;

FIG. 4 is a simplified view, in vertical section, of apparatus employedin carrying out another mode of the process of this invention, showingplastic sheet in position for molding prior to engagement of the diemembers;

FIG. 5 is a simplified view of part of the apparatus illustrated in FIG.1, showing a multiple number of sheets in position for molding;

FIG. 6 is a view of the apparatus of FIG. 5 after the sheets have beenshaped;

FIG. 7 is a simplified view of part of the apparatus of FIG. 1,utilizing a modified male die in the molding of a multiple number ofsheets for the production of multiwall articles; and

FIG. 8 is a view of the apparatus of FIG. 7 after the sheets have beenshaped.

The apparatus employed in the practice of this invention may besubstantially the same as that shown for pneumatic molding of articlesfrom agglomerated fibrous materials as described in U.S. Pat. 1,260,002to Lanhofier, or in forming sheet metal between a die and a confinedbody of yieldable material, such as a rubber pad, as described in US.Pat. 2,190,659 to Guerin, and in Section 13 of Die Design Handbook,"ASTE, McGraw-Hill Book Company, Inc., 1955.

Turning now to the drawing, in which like symbols refer to like parts inthe several figures, the apparatus illustrated in FIGS. 1, 2 and 3 ispart of a conventional hydraulic press, utilizing a die system accordingto this invention. The die set comprises a rigid male die member 1 and ayieldable female die member 2.

Die member 1 may be constructed of metal or other conventionalnon-yielding material. It is mounted on a block 3 and base 5 belowpress-ram 7 of a single-acting hydraulic die-press, or on the lower ramof a doubleacting die press. Block 3 and its base 5 together containsuitable form-ejecting means, here illustrated by pneumaticallyactivated piston 6.

Die member 2 is typically made of rubber or of a rubber-like material orcomposition. This includes any elastomeric materials, such as vulcanizedrubber and synthetic elastomers, which have sufficiently rigidity toretain their shape when not under pressure and which are within theirelastic limits, capable of flow similar to that of a liquid, when underpressure during the forming process. The chemical composition of theelastomer is of no consequence from the point of view of this invention,provided it has the desired physical characteristics, is not degraded atthe desired operating temperature, and is chemically inert to thethermoplastic material being formed. A nubber composition resemblingtire stock is suitable. Polyurethane rubber may be employed for longerlife. Life of the rubber can also be prolonged by using a thin coversheet to take the maximum wear and replacing this periodically. Thecover sheet may also be rubber or similar suitable composition. Thevolume of elastomeric material in the yieldable die member should be atleast about three times the volume of the part being formed.

Die member 2 is confined in housing 4 which is suitably attached to thehead of ram 7, and which confines the elastomer whenthe dies areengaged, and prevents its lateral expansion. Air passage '9 throughhousing 4 and elastomer 7 permits removal of air from the confined spacecreated when the ram is lowered and breaking vacuum after a part hasbeen formed.

After plastic sheet is mounted on die 1 as shown in FIG. 1, the ramcarrying yieldable die member 2 moves down into engagement with rigiddie 1. The fully engaged dies are illustrated in FIG. 2. As shown, theupper part of block 3 has entered the lower opening of housing 4 andforms part of the space in which yieldable die 2 is confined. The diesare retained in engagement for a brief period, typically from 1 secondto about 1 minute, sufficient to relieve stress in the molded sheet. Inany production run, a few simple tests at different times will permitdetermination of the minimum required stress relief period. At the endof this period, ram 1 is withdrawn and the shaped form ejected as shownin FIG. 3.

An alternative method is illustrated in FIG. 4. In this method, male diemember 11, schematically shown as a unit, may, for example, have thesame construction as die 1, block 3 and base 5 of FIG. 1. In place ofthe solid elastomer block 2 of FIG. 1 as yieldable die' member, there isemployed a cushion 12 of liquid, such as water, confined between housing4 and a confining bag or sheet 13. Sheet 13 preferably is made of anelastomer which has the properties described in connection with diemember 2 of FIG. 1. The material of sheet 13 is selected to providesufficient friction between its outer surface and the surface of billet10 which it contacts, to cause the billet to be deformed under tension,as described elsewhere in this specification. Operation of the method ofthis invention with the apparatus of FIG. 4 is identical in principlewith operation using the apparatus of FIG. 1 as described above. Inthis, as in the other described modes, the pressure of the yieldablemass against one side of the sheet being shaped and of the contactsurface of the rigid die on the other side is sufficient to hold thesheet in place without need to secure the rim of the sheet againstmovement by a clamping device.

FIGS. 5 and 6 illustrate the simultaneous shaping of a multiple numberof sheets 10A, 10B, and 10C, showing only the rigid male die 3, prior toengagement of the dies in FIG. 5 and after completion of shaping in FIG.6. In this method of operation, each sheet is formed into a separatearticle. The sheets may be of identical or different composition. Thedimensions of the articles will obviously vary from each other, thediameter of 10B being greater than that of 10A, etc. The method issuitable where such variation is not objectionable.

FIGS. 7 and 8 illustrate the formation of a single article consisting oftwo layers of different plastics, 10B and 10F. In this case, male die 13has a sufficient undercut at some location to provide a lockingengagement between the layers of plastic in the finished article.Plastics normally have sufficient elasticity to permit ready ejectionfrom a form having a slight undercut. The blank consisting of sheets 10Band 10F may be a laminate, but it is preferred to employ two separatesheets.

Preferred embodiments of this invention have been described. The methodis, however, capable of numerous modifications While remaining withinthe scope of this invention. For example, it is not essential that therigid die or its base be capable of sliding into the housing of theyieldable die. In an alternative mode, one or several rigid forms areplaced on the bottom plate or bottom ram of a press, covered by aplastic sheet to be formed, and. an elastomer pad confined againstlateral movement is brought down by the opposing ram and pressed downover the sheet and onto the bottom plate or ram.

The shape, bulk, hardness and distribution of the yieldable die can bevaried or controlled to provide variable unit pressures over selectedareas of a blank or form, thus permitting some localized control ofshape and thickness of the formed article. Many other modificationswithin the scope of this invention will occur to persons skilled in thisart.

The method of this invention is particularly useful in the forming ofarticles from sheets of polymers which have a high degree of springbackwhen molded between matched rigid dies. Polypropylene of commerce,exemplified by isotactic polypropylene, is such a polymer, Another ispolyvinyl chloride. The invention is particularly suitable for polymerswhich normally exhibit relatively high crystallinity and thus arecharacterized by a relatively sharp melting point and by theircapability of being strengthened by orientation. However,non-crystalline polymers can also be formed by the method of thisinvention. The method can, for example, be successfully practiced withsheets of polypropylene, polyvinyl chloride, cellulose acetobutyrate,polystyrene, ABS-polymer (a copolymer of acrylonitrile, butadiene andstyrene), and acrylic polymer (Plexiglas, a polymethylmethacrylate) anda polycarbonate. The invention is also useful in forming either highdensity or low density polyethylene and in general, thermoplasticpolymers such as polyolefin, cellulose esters, poly(vinyl benzenes) andtheir copolymers such as ABS, acrylics and polycarbonates. Thethermoplastic sheets for use in this invention may be polymercompositions containing a single polymer or copolymer or blends ofpolymers or copolymers, and may contain such conventionally addedmaterials as stabilizers against thermal or actinic degradation, dyes orpigments, impact improvers such as rubbers, and the like. The presenceof fillers of various types, including powdered and fibrous materials,e.g., talc, glass fibers or asbestos, can be tolerated but is generallynot preferred. Long-fibered fillers are generally undesirable.

The method of this invention is useful with polymers having molecularweights highenthan can be accomodated in conventional moldingoperations, e.g., weight average molecular weights above about1,000,000, but it is equally applicable to solid thermoplastic polymercompositions in the molecular weight range conventionally employed inproduction of plastic articles, e.g., weight average molec-' ularweights above about 10,000, the lower limit being dilferent fordifferent polymers.

In one special mode of practicing this invention, multiwall articles areformed by molding two or more superimposed sheets dilfering incomposition. This may comprise dilferent formulations of a single typeof polymer or sheets made up of different polymers. In this manner,desirable combinations of properties may be obtained in articles whichcould not be formed by thermoforming methods when the differentcompositions diifer significantly in melting point or softening range.For example, a clear, glossy acrylic sheet, such as apolymethylmethacrylate type may be used as the outer layer of an articleformed of a thicker sheet or a more inexpensive, less impact-andultraviolet-resistant polymer composition, e.g., polystyrene.

In producing such multi-wall composite articles, it is generallydesirable to utilize compositions in such a Way that the inner layer ofthe article, i.e., that nearest the male die, has the greatercoefiicient of thermal expansion. It is also desirable to provide in themold sufiicient indentations, undercuts, projections or similarirregularities of shape to cause the multiple layers to be boundtogether by the resulting deformation. The undercut should not exceed involume one fourth of the part volume to avoid permanent deformation ofrubber used as yieldable die member.

In another mode of practicing this invention, deformable sheets of metalmay be combined with thermoplastic sheets into composite articles. Forexample, a metal foil may be placed between two thermoplastic sheets toform a composite having an inner metal layer to provide articles havingspecial decorative or electrical properties. Metal sheets may also becombined with thermoplastic sheets in the same manner that combinedarticles consisting of two thermoplastic sheets are formed.

The method of this invention is applicable to sheets at least about 0.01inch thick. It is preferred to apply it to sheets at least about 0.075inch thick. There is no specific upper limit of thickness, provided thedies are appropriately proportioned and the press sufliciently powerful.Sheets of 1 inch and 1 /2 inch thickness have been successfully moldedby this method, and it can be applied to sheets of 2 to 3 inchthickness.

In simultaneously molding a stack of several sheets, there is no upperlimit on the number of sheets or the thickness of the stack, but athickness of 2 to 3 inches will ordinarily not be exceeded.

The method of this invention applies to the forming of articles ofsubstantially uniform thickness because sheet of uniform thickness isemployed as the blank and there is no substantial redistribution of thepolymer mass. The method applies particularly to relatively shallowdraw, i.e., forming of sheet into articles having a depth to diameterratio not exceeding about /2 :1; designs in which this ratio is up toabout 0.25:1 are particularly suitable. In designing dies for thisprocess, it is desirable to have the ratio of curvature within the partat least equal to the initial sheet thickness.

Depending on the selected conditions, it is possible to mold patternedsheet, e.g., embossed sheet, with retention of the pattern. This isachieved by placing the patterned side toward the yieldable die andkeeping the temperature of the sheet and the forming pressure employedboth in the lower part of the suitable range. It is also possible toemboss designs into the formed parts by forming smooth sheet attemperatures and forming pressures in the high part of the suitablerange.

The pressure required in pressing sheets according to this invention issubstantially higher than pressures in thermoforming processes, but isstill sufficient to permit operation of the process in presses ofconventional industrial size, e.g., from 200 to 10,000 tons, dependingon the size of the articles to be shaped. Forming pressures may be aslow as about 200 p.s.i. and are generally in the range from 1000 to 2000p.s.i. Higher forming pressures may be employed.

The temperature of the thermoplastic sheet must be above the glasstransition temperature of the polymer. This is a property which isreported in the technical literature for most polymers or which can bedetermined by known methods. The temperature must be below the meltingpoint for crystalline polymers and, for all polymers, below thethermoforming temperature as herein defined, i.e., that temperature atwhich the sheet would drape or sag under its own weight, or at which itcould be deformed by a pressure of about one atmosphere. Elongation ofthermoplastic sheet increases with increasing temperature; substantialelongation before break 8 facilitates the method of this invention, andit is therefore desirable to operate on warm sheet. However, retentionof strength at the shaping temperature is also important to obtain theadvantages of this invention, and for this reason thermoformingtemperatures are avoided. For use in the method of this invention, sheetis generally not heated above the temperature at which its tensilestrength is still substantially above p.s.i. e.g., as high as 1000p.s.i. or more.

For polypropylene a suitable temperature is in the range from to 330 F.and the preferred range is from 280 to 325. For other crystallinepolymers, temperatures are also within 50 F. of their crystallinemelting point, but sufficiently low to retain the desired tensilestrength. For example, linear polyethylene, which melts at about 270 F.,is preferably formed at temperatures from 220 to 250 F.

Dwell time after the molds have been brought into contact need be onlysuflicient to assure stress relaxation in the molded article. This iseasily determined by molding several articles at increasing dwell timesand otherwise fixed conditions, and measuring springback in the formedarticles. In general, dwell times will be in the range from one secondto one minute.

The temperature of the dies may have some effect on the requiredconditions for successful operation. For example, dwell time requirementmay be less when the tool is substantially cooler than the sheet. Tooltemperatures may vary from about room temperature to upper workingtemperature limit for the polymer being formed, as defined above. As anexample, tool temperatures of about 150 F. were found desirable forshaping polypropylene sheets which were at a temperature of about 300 F.when placed in the press.

I claim as my invention:

1. The method of producing three-dimensionally shaped thermoplasticarticles from flat sheets of thermoplastic polymer composition at aworking temperature at which the tensile strength of said sheet issubstantially above 100 p.s.i., said working temperature being above thepolymer glass transition temperature and below the deformationtemperature at which said sheet is deformable by application ofsubstantially atmopsheric pressure, which comprises (a) placing a flatbody comprising at least one such sheet, at said working temperature,between a nonyielding three-dimensional die and a laterally confinedbody of yieldingly deformable material having a substantially greatervolume than said flat body and having an elastomer surface adapted tocontact the surface of said flat body which it faces, said die and bodyof yieldingly deformable material being adapted to be forced toward eachother by mechanical force,

(b) while said flat body is unsecured between said die and deformablebody and is at said working temperature, forcing said confined body ofyieldingly deformable material and said die toward each other bymechanical force to cause the surface of said body of yieldinglydeformable material to contact substantially the entire surface of saidflat body, said force being sufficient to provide a forming pressure ofat least about 200 p.s.i. and to cause said mass to deform said flatbody to conform to the shape of said die, whereby parts of the shapedarticle are thinned during said deformation and are strengthened by theresulting orientation,

(0) retaining said flat body and die in said forced engagement for asufficient time to relieve stress in the resulting shaped body, and

(d) removing from said die the resulting article having substantiallythe dimensions it had while in said forced engagement with said die.

2. The method of producing three-dimensionally shaped thermoplasticarticles from flat sheets of thermoplastic polymer composition at aworking temperature at which the tensile strength of said sheet issubstantially above 100 p.s.i., said working temperature being above thepolymer glass transition temperature and below the deformationtemperature at which said sheet is deformable by application ofsubstantially atmospheric pressure, which comprises (a) placing a flatbody comprising at least one such sheet, at said working temperature,between a nonyielding three-dimensional die and a laterally confinedmass of solid elastomer having a substantially greater volume than saidflat body, said die and mass of elastomer being adapted to be forcedtoward each other by mechanical force,

(b) while said fiat body is unsecured between said die and said mass ofsolid elastomer and is at said working temperature, forcing said mass ofsolid elastomer and said die toward each other by mechanical force tocause said mass of elastomer to contact substantially the entire surfaceof said flat body, and force being sufiicient to provide a formingpressure of at least about 200 p.s.i. and to cause said mass to deformsaid flat body to conform to the shape of said die, whereby parts of theshaped article are thinned during said deformation and are strengthenedby the resulting orientation,

(c) retaining said flat body and die in said force engagement for asulficient time to relieve stress in the resulting shaped body, and

(d) removing from said die the resulting article having substantiallythe dimensions it had while in said forced engagement with said die.

3. The method according to claim 1 wherein said flat body is a singlesheet having a thickness in the range from 0.075 inch to 2 inches.

4. The method according to claim 1 wherein said flat body consists of alaminate of at least two sheets of different polymers, and said workingtemperature is above the highest of the respective glass transitiontemperatures and below the lowest of the respective deformationtemperatures.

5. The method according to claim 1 wherein said fiat body consists of anumber of superimposed sheets.

6. The method according to claim 1 wherein said flat body consists of atleast two sheets of different polymers, said working temperature isabove the highest of the respective glass transition temperatures andbelow the lowest of the respective deformation temperatures, and saiddie has a sufficient amount of undercut to cause the shaped sheets inthe finished article to remain firmly attached to each other.

7. The method according to claim 1 wherein said polymer is characterizedby a crystalline melting point, said working temperature is within 50 F.of but below said melting point, said period of retention of sheet anddie in forced engagement is in the range from about 1 second to about 1minute, and the dimensions of the resulting shaped articles aresubstantially identical to the dimensions of said articles while inforced engagement with said dies.

8. The method according to claim 1 in which said polymer ispolypropylene.

9. The method according to claim 1 in which said polymer is polystyrene.

10. The method according to claim 1 in which said polymer is polyvinylchloride.

11. The method according to claim 2 wherein said flat body is a singlesheet having a thickness in the range from 0.075 inch to 2 inches.

12. The method according to claim 2 wherein said fiat body consists of alaminate of at least two sheets of different polymers, and said workingtemperature is above the highest of the respective glass transitiontemperatures and below the lowest of the respective deformationtemperatures.

13. The method according to claim 2 wherein said flat body consists of anumber of superimposed sheets.

14. The method according to claim 2 wherein said flat body consists ofat least two sheets of different polymers, said working temperature isabove the highest of the respective glass transition temperatures andbelow the lowest of the respective deformation temperatures, and saiddie has a sufiicient amount of undercut to cause the shaped sheets inthe finished article to remain firmly attached to each other.

15. The method according to claim 2 wherein said polymer ischaracterized by a crystalline melting point, said working temperatureis within 50 F. of but below said melting point, said period ofretention of sheet and die in forced engagement is in the range fromabout 1 second to about 1 minute, and the dimensions of the resultingshaped articles are substantially identical to the dimensions of saidarticles while in forced engagement with said dies.

16. The method according to claim 2 in which said polymer ispolypropylene.

17. The method according to claim 2 in which said polymer ispolystyrene.

18. The method according to claim 2 in which said polymer is polyvinylchloride.

References Cited UNITED STATES PATENTS 2,351,475 6/ 1944 Berger 264320X2,362,672 11/ 1944 Sloan. 2,385,083 9/1945 Kemerer 1856 2,390,80312/1945 Marschner 2643 16X 2,890,541 6/1959 Siegel 264249 3,184,524 5/1965 Whiteford 264-320X 3,319,295 5/1967 Jones-Hinton 1819 3,384,6955/1968 Murray 264322X 3,470,291 9/ 1969 Johnson 264-292 FOREIGN PATENTS944,458 6/ 1956 Germany. 739,436 10/ 1955 Great Britain.

OTHER REFERENCES Plastics Engineering Handbook, Reinhold Pub. Co., N.Y.,1954, p. 182 relied on.

ROBERT F. WHITE, Primary Examiner R. R. KUCIA, Assistant Examiner U.S.Cl. X.R. 264-266, 313, 322

